High Gain Mode 2 would be the most reliable.
Gain is more linked to power consumption and start time than accuracy.
Accuracy is determined mostly by the grade of the resonator.
I can't speak to this particular chip, however I've seen reliability problems over the years with using low power (low gain) oscillators. They don't always start reliably across voltages and temperature ranges.
If your resonator has internal caps, disable any built-in caps that the chip may offer.